Method and apparatus for continuously extruding single-wall pipe of plastics or other mouldable material

ABSTRACT

The invention refers to a method and apparatus for manufacturing a single-wall pipe, tube or like conduit (9,44) having smooth inner surfaces and a mechanical-strength enhancing pattern of walls, flanges or ridges on the outer surface thereof, the conduit being made of plastics or like extrudable material.

The present invention relates to a single-wall pipe, tube or likeconduit of a heat-mouldable or pressure-mouldable material, the conduithaving smooth inner surfaces and a mechanical-strength enhancing patternon the outer surface thereof. The invention further relates to a methodand apparatus for manufacturing such conduits.

Such smooth inner-wall conduits are used in underground trenches as ameans for protecting electric cables, in heat-exchangers etc. where thepresence of irregularities, such as corrugations, on the inner surfacesof the conduits may create difficulties and/or in the cases where theconduit is subjected to relatively high mechanical streeses fromexternally of the conduit. A conduit manufactured in accordance with themethod of the present invention can be used to advantage in those caseswhere a conventional corrugated or non-corrugated, smooth, conduit couldotherwise be used, since a conduit produced in accordance with theinvention for a similar throughflow area and external stresses, ischeaper to manufacture and lighter in weight, due to the fact that,inter alia, it incorporates less material per unit length.

Smooth inner-wall conduits having flanges or like protruberances ontheir outer surfaces are known to the art, inter alia manufactured bycontinuous extrusion. One method of manufacturing such conduits isdescribed in Swedish Patent Application No. 7407555-7, according towhich a conventional corrugated conduit is formed in a conventionalmanner by pressing a plasticized or molten material by means of airunder positive or negative pressure against a moulding tool having theconfiguration desired of the moulded conduit, there being obtained aconduit which is patterned on both the inside and the outer sidethereof. Further material is then introduced into the inside of thecorrugated conduit, through an extrusion nozzle. This material forms asmooth sleeve whose outer surface bonds to the inner surface of theconduit. The conduit thus produced is not, however, a single-wallconduit, and the method by which the conduit is manufactured isencumbered with several disadvantages.

According to another method known for example from Swedish Pat. No.7314295-2, material extruded by an extruder is fed to the space betweentwo corrugated mould-jaw or die chains and is pressed into thecorrugations of the dies with the aid of a conical mandril, the greatestexternal diameter of which determines the inner diameter of the conduitunder manufacture. It is difficult with this method to producesingle-wall conduits of good all-round quality. Because the material ispressed into the corrugation-forming or flange-forming cavities in thedies with the aid of an inflexible mechanical mandril, instead of withthe aid of resilient prssurized air, it is difficult to fill all thecavities uniformly, so as to obtain fully formed corrugations orflanges. When the material is pressed into the aforesaid cavities,pockets of air readily form at the bottoms thereof, these pockets beingempty of filling material. This is one of the reasons why incomplete ornon-uniform flanges are formed. Furthermore, strong restrictions areplaced on the height and form of the flanges, since it must be possibleto release the moulded conduit from the moulding jawa or toolsdownstream of the moulding line in which the conduit is produced.

Light-weight conduits or pipes may be designed in the two principallydifferent ways described above in order to achieve low weight andreasonable strength. However, different, in certain respects entirelydifferent, products are obtained. In one case a single-wall pipecomprising outer, strengthening flanges is obtained while in the othercase a double-wall pipe having hollow parts is obtained. It is obviousthat the egg-shell design of the double-wall pipe makes the pipe lessresistent to blows etc. than the flanged, single-wall pipe. Thus thesingle-wall pipe is especially suitable for pipes made of hard, brittlematerials. The double-wall pipe comprising hollow parts is suitable forpipes made of soft and tough materials having good resistance to blowsetc. even when very thin walls are used.

Another important difference between the flanged, single-wall pipe andthe double-wall pipe having hollow parts, is that the insulatingproperties of the double-wall pipe are much better than those of thesingle-wall pipe. This is, of course, a great disadvantage if the pipesare meant for heat-exchanging purposes.

The object of the present invention is to provide a single-wall pipe,tube or like conduit or tubular element of the kind indicated above, theconduit being arranged so as to retain all the important advantagesafforded by known conduits while offering new possibilities to adapt thematerial of the conduit to the use of the conduit. These objects areachieved by means of conduits formed in accordance with the principalsof the invention. According to one embodiment of a conduit according tothe invention a pipe extremely suitable for heat exchangers is provided,the pipe having an outer part made of a highly modified plasticsmaterial providing strength and good thermal conductivity and a thininner part made of unmodified material providing resistance tochemically agressive substances.

Another object of the present invention is to provide a method formanufacturing conduits according to the present invention. However,single-wall conduits of known kind may be manufactured by means of themethod. A further object is to provide a method, which solves the basicproblems encountered in known methods. These objects are achieved bymeans of method steps occuring in accordance with the principals of theinvention. The present invention also relates to apparatus for carryingout the method according to the invention. Apparatus for carrying outthe invention is provided in accordance with the principals of theinvention.

When practicing the method according to the invention conduits can bereadily released from the relevant forming jaws or dies downstream ofthe forming line incorporating said apparatus for carrying out theinvention.

So that the features of the invention will be readily understood and theadvantages afforded by the invention become more apparent, a number ofembodiments of apparatus for carrying out the method according to theinvention will now be described in more detail with reference to theaccompanying drawings, in which

FIG. 1 is a plan view of a schematically illustrated apparatus forcarrying out the method according to the invention,

FIG. 2 is a longidutinal sectional view of an extrusion nozzle accordingto the invention having an end connected to an extruder,

FIG. 3 is a longitudinal sectional view according to FIG. 2 through amoulding line according to the invention,

FIG. 4 illustrates a quarter of a sectional view taken on the line A--Ain FIG. 3,

FIG. 5 illustrates a spacer according to the invention, seen radiallyin, for example, FIGS. 3 and 4,

FIG. 6a illustrates moulding jaws according to the invention seen intheir direction of movement in the moulding line, the figure indicatingthe displacement of the jaws when releasing the formed conduitdownstream of the moulding line.

FIG. 6b illustrates the arrangement of FIG. 6a seen from above in FIG.6a, where for the sake of clarity certain elements have been omitted,

FIG. 7 is a schematic sectional view of the external cylindrical surfaceof a first embodiment of a conduit produced by a method according to theinvention,

FIG. 8 is a cross-sectional view of a conduit substantially according toFIG. 7,

FIG. 9 is a schematic cross-sectional view of the external cylindricalsurface of a second embodiment of a conduit produced in accordance witha method according to the invention,

FIG. 10 is a cross-sectional view of a conduit substantially accordingto FIG. 9,

FIG. 11 is a schematic cross-sectional view of the external surface of athird embodiment of a conduit produced by a method according to theinvention, and

FIG. 12 illustrates in cross-section a conduit substantially accordingto FIG. 11.

The apparatus illustrated in FIG. 1 includes two endless chains ofmould-jaws 1,2 constructed of mould-jaw parts 3. The mould-jaw chains1,2 are arranged to move through a given distance in close proximity toone another, while being guided in a firmly mounted channel-like guiderail 4 and channel-like guide rail 5, which is preferably arranged forlimited axial movement and which is urged towards the guide rail 4preferably by means of thrust springs 6 mounted between the guide rail 5and a firmly mounted abutment means 7. Arranged in the region where themould-jaw parts run together is an extrusion nozzle 8, which isconnected to at least one extruder (not shown) arranged to accomodateand to feed highly fluid or plasticized material, such as a plasticsmaterial or like extrudable material. The moulded conduits aredischarged from the illustrated apparatus at the location thereof wherethe mould-jaw parts move away from one another, as illustrated by theconduit 9.

FIG. 2 illustrates the end of the extrusion nozzle 8, which can beutilized in a one extruder process, where material 10 from the extruderis introduced into the nozzle through a passage 11 in a connector 12.The nozzle is secured in an axially extending recess 16 in the connector12 via a flange ring 13 of an outer jacket 14. Arranged radiallyinwardly of the jacket 14 and in the proximity thereof is a first, outermandril 17 or like device in the form of a tubular sleeve, where a firstchannel 18 of circular, substantially tubular cross-section forsupplying material 10 is formed between the outer jacket 14 and themandril 17. A second, inner mandril 19 or like device is providedinwardly of said first mandril 17, there being provided a second channel20 of circular, tubular cross-section for supplying material 10.

The channels 18,20 connect with the extruder in said connector 12, wheresaid outer jacket 14 is secured by means of a flange ring 21 in therecess 16 inwardly of the flange ring 13 of the outer jacket 14, andwhere the inner mandril 19 is provided with screw threads and is firmlyscrewed into an end piece 22 forming part of the outer mandril 17.Extending axially in the inner mandril 19 is a channel 23 intended,inter alia, for supplying pressure medium. As will be seen from thefigure, setting screws 24, suitably four in number, are provided fordisplacing primarily the outer mandril 17 radially in the recess 16, tocenter said mandril.

FIG. 3 illustrates schematically the moulding line in which the material10 is actually moulded to form a conduit. The ends 25,26 of the mandril17,19 respectively protrude inwards at a part of the moulding line inwhich respective mould-jaw parts have just joined one another. Theconduit to be moulded in the moulding line is intended to be produced inat least two stages. In accordance with the invention, when seen in thedirectin of movement of the material 10, identified by arrows 27 in FIG.3, the nozzle includes a first separate location arranged before asecond separate location, both for supplying material to the mouldingline. In accordance with one preferred embodiment of the invention, theaforesaid first channel 18 supplies material at said first location toan annular space 29 formed between a tangentially extending recess 28 inconnection with the end 25 of the outer mandril 17 and the mould-jaws 3.For the purpose of supplying material to said second location, theaforesaid second channel 20 discharges into a space 30 located at theends 25,26 of the mandril 17,19, this space 30 being defined in themovement direction of said material by a smoothing mandril 31 locatedwithin the moulded conduit and the ends 25,26 of the mandril 17,19facing the smoothing mandril 31, and is defined peripherally by themould-jaw parts 3.

In order to fill all intended cavities, the material supplied to thefirst and the second locations is pressed against the mould-jaw parts 3,against mutually opposing surfaces in the moulding line, by means ofdevices which, according to one embodiment of the invention, comprisemeans for extracting air from the cavities of the mould-jaws, thesecavities being intended to be filled with plasticized material 10, andmeans for supplying a pressure medium, preferably pressurized air orsome other gas, to said cavities or spaces 29,30.

As illustrated, passages 32 are provided which extend from the channel23 in the inner mandril 19, through the inner and the outer mandrilsubstantially radially, and which discharge into the cavity 29. See inthis respect FIGS. 3 and 4. In this embodiment, the passages 32 passthrough the circular channel 20 in lead-throughs or bushes 33 arrangedin the walls thereof, these bushes simultaneously forming spacers 33, bymeans of which the inner mandril 19 is held centrally guided in relationto the outer mandril 17. Suitably, additional spacers 33' are alsoprovided.

The reference 34 identifies extremely fine passages for extracting airby suction, these passages 34 preferably opening into the bottom parts35 of the mould cavities 36 of the jaws 3, these mould cavities formingthe external surface of the conduit under manufacture. Means forproducing a vacuum in the passages 34 will be described hereinafter. InFIG. 3 the reference 37 identifies a pipe or like membere which extendsin the direction of movement of the material 10 from the end 26 of theinner mandril 19 and here forms a continuation of the pressure-mediumsupply channel 23. The pipe 37 extends axially through the cavity orspace 30, and further through the smoothing mandril 31, forming asupport therefor, and through a cooling space 38, downstream of themandril 31, and continues through the apparatus in a manner hereinafterdescribed. That section of the pipe 37 located in the cavity 30 isprovided with holes or, nozzles 39 or the like through which pressuremedium is supplied to the cavity 30 for forcing the material supplied atsaid second location towards the amterial supplied at the first locationand towards the mutually facing surfaces of the mould jaws 3 in themoulding line. The aforesaid pressure medium also cools the mouldedconduit from the inside.

The smoothing mandril 31, which defines the other end of the actualmoulding line has a softly rounded convex ingoing end 40 which faces themandril 17,19, and an outgoing end 41, and comprises between said ends40,41 a substantially cylindrical portion 42, along which the materialpressed against the mould-jaws is intended to run.

The smoothing mandril 31 includes means for heating the same to atemperature of such magnitude that the material on the inner surface 43of the conduit 44 formed in the mould line is held in a molten orplasticized state to an extent such that the inner surface 43 will besubstantially smoothed by sliding against the mandril 31. FIG. 3illustrates electrical heating devices 45, in which the supply ofelectricity is effected through the pipe 34 and the channel 23.

Arranged in the colling space or, the cooling region 38, immediatelydownstream of the smoothing mandril 31 in the direction of movement ofthe conduit 44 are means for supplying a coolant to the inner surface 33of the conduit 44, these means in the illustrated embodiment comprisingnozzles or orifices 46, in the pipe 37, through which air or some othersuitable gas is supplied and preferably held at a given pressure aboveatmospheric pressure or ambient pressure. Downstream of the coolingregion 38 there is provided on the inside of the conduit 44 andpreferably adjacent the pipe 37 one or more, preferably resilientsealing washers 47, which are arranged to abut the inner surface of theconduit and contribute to maintaining said over pressure, by obstructingthe outflow of coolant.

The surfaces 48 of the mutally facing mould-jaws 3 in the moulding linehave a configuration which corresponds to the outer surface 49 of theconduit 44 under manufacture, said configuration thus forming therecesses 36. In the embodiment illustrated in FIG. 3, a substantial partof the conduit thickness comprises radially directed, relatively thinwalls 50, forming therebetween outwardly opening recesses, ashereinafter described, when seen in section in FIG. 3.

In accordance with one embodiment of an apparatus according to theinvention, the mould-jaw parts 3, which are driven by chains 51 or thelike as schematically illustrated in FIG. 1, are divided into twopreferably symmetrical section 3', 3", best seen at FIG. 6a in thelongitudinal direction of the mould line, said mould-jaw parts 3 beingarranged to form a quadrisectional mould-jaw arrangement in the mouldingline, The aforesaid chains 51 or like devices are driven in a knownmanner, by means of wheels or rollers 52 or the like (FIG. 1). Accordingto the invention, there is provided at each jaw part 3 two chains 51',51", wherewith a chain is arranged at each jaw section 3', 3", andwherewith a drive wheel 52', 52" or the like is provided for each chain51', 51" as illustrated in FIG. 6a. The aforesaid chains 51', 51" andwheels 52', 52" are arranged so that at least one of said jaw parts 3',3", preferably both of said parts, is, or are, capable of being moved toa limited extent towards and away from respective opposing jaws parts orsection, substantially parallel with the axis of rotating of respectivewheels 52', 52", said axis preferably being common to both of saidwheels, and substantially parallel with the line 53 or the like alongwhich the jaw sections 3 are intended to join one another in themoulding line. The reference 54, 55 identify guide means which maycomprise, for example, at least one guide pin 54 arranged adjacent theone jaw section 3' and having square or elongated rectangularcross-section tapering from said jaw section 3', and a recess 55arranged in the jaw section 3" and corresponding in form to the pin 54,said means 54, 55 being arranged to guide the jaw sections 3', 3", insaid movement towards and away from one another.

Arranged in the vicinity of the part of the moulding line at which theconduit leaves said line is a lifting zone 56,57, FIG. 1, where theguide means 4,5, for the purpose of drawing the mouldjaws free from theformed conduit, are arranged so that the mould-jaw sections 3 are liftedfrom the conduit in a direction away from one another, at the same timeas the sections 3', 3" of each jaw part 3 are moved away fromoneanother, the lifting and displacing movements being coordinated withone another so that the four jaw sections 3', 3" encircling the conduitare displaced symmetrically in radial directions through substantially90° in relation to one another, as shown schematically by the arrows 58in FIG. 6a. Subsequent to releasing the conduit, the jaw parts 3 areintended to be swung away from said conduit and respective jaw sections3', 3" are moved towards one another. The lifting of jaw parts 3 and thedisplacement of jaw sections 3', and 3" is arranged to be effected in asuitable manner, for example as schematically shown at FIG. 6b by meansof guides 59 arranged to co-act with grooves 60 in said parts 3, and ameans 61 having a wedge shape in the longitudinal direction of themoulding line and arranged to activate, for example, the chains 51',51". The jaw sections 3', 3" can also be guided by means of guides orthe like arranged to co-act with said sections.

The various embodiments of conduits illustrated in FIGS. 7-12 areintended for manufacture by means of the method according to theinvention and are distinguished by the fact that external and closelyadjacent arranged recesses 62 extend from the outer cylindrical surface44' fo the conduit 44, substantially radially inwards and form areinforcing system of external,preferably thin intermediate walls,flanges or ridges 63, which strengthen the conduit. In many instances itis preferably that the geometric extension of each recess, for examplein the peripheral and longitudinal direction of the conduit, is of thesame order of magnitude as the wall thickness of said conduit.

In the embodiments illustrated in FIGS. 7 and 8 the recesses 62 are ofsubstantially square cross-section at right angles to the radialdirection of the conduit. According to one embodiment, intermediatewalls 63 have the form of helices, right-hand helices and left-handhelices. This embodiment is illustrated in the section taken on the lineB--B in FIG. 7, said section extending circumferentially orlongitudinally of the conduit, and in FIG. 8. According to anotherembodiment, the intermediate walls 63 extend circumferentially andlongitudinally of the conduit, and a section taken along line C--C inFIG. 7 thus corresponding to the longidutinal or circumferentialdirection.

In the embodiments illustrated in FIGS. 9 and 10, when seen at rightangles to the radius of the conduit, the recesses 62 have thecrosssectional shape of a regular hexagon.

In the embodiments illustrated in FIGS. 11 and 12, the recesses 62 havea substantially circular cross-section when seen at right angles to aradius of the conduit.

Thus, in the case, inter alia, of thick-walled conduits produced bymeans of the method according to the invention, the intermediate walls63 between the recesses 62 are arranged to contribute substantiallytowards strengthening the conduit with regard to stresses created byinternal and external pressures. In this respect, the radial height ofthe intermediate walls forms a substantial part of the total wallthickness of the conduit, for example from one half to two-thirds saidthickness.

For the purpose of producing conduits of the kind illustrated in FIGS. 7and 8, the mould-jaw parts are designed so that continuous channels areformed in the moulding line, these channels extending in the form ofleft-hand and right-hand helices, or may extend longitudinally andcircumferentially, thereby to provide a diamond-shaped network ofintermediate walls.

For the purposes of producing conduits of the kind illustrated in inFIGS. 9-12, the mould-jaw parts 3 comprise outwardly projecting pegswhich form the aforesaid recesses 62, the pegs having a crosssectionalshape corresponding substantially to a regular hexagon or a circlerespectively.

In order to facilitate removal of the conduit from the mould-jaws, themould-jaw parts 3 may be designed so that the intermediate walls 63taper outwardly slightly, and/or are provided with a given clearance orrelief.

As beforementioned, in order to facilitate filling of the recesses inthe mould-jaws with moulding compound or, in certain cases, to enablethese cavities to be filled, the air present in primarily the bottoms ofthe recesses is evacuated therefrom by suction. To this end, there isprovided a tunnel 64 or the like, as indicated in FIG. 1, in thevicinity of the entry to the moulding line. Air is extracted through thetunnel 64 in a suitable manner, so as to produce a tangible sub-pressureor vacuum. The mould-jaws are intended to pass through the tunnel 64,wherewith suitable sealing is effected between the walls of the tunneland the mould-jaws.

Of course, as indiated earlier, more than one extruder may be provided.In such embodiments more than one extruder conduits according to theinvention may be produced which conduits comprise more than onematerial. Thus a conduit may be produced which is of a compositestructure and comprises two different concentric parts 44", 44'", (FIG.8). For example, an outer part may be made of a material with certainproperties, which material is provided from a first extruder throughsaid first channel 18, and an inner part made of another material withcertain other properties, which material is provided from a secondextruder through said second channel 20. The invention is, of course,not restricted to an extruder nozzle according to FIG. 2 but the nozzleshould be adapted to the extruder, the number of extruders etc. in a waywhich is obvious.

Thus, it will be understood that in the case of two extruders, one for acertain material and one for another material, the embodiments shown inFIGS. 7-12 comprise two materials with each material constituting aconcentric part of the conduit. The distribution of these parts in theradial direction of the conduit may, of course vary e.g. betweenconduits for different purposes. Thus, embodiments are producable inwhich the conduit comprises an outer part made of one material andcomprising said pattern of walls, flanges or ridges, the conduit furthercomprising an inner part made of another material. Such as embodiment isillustrated by means of the dashed circumferential line 65 in FIG. 8.

The working mode of the appartus according to the invention and themanner in which the method according to the invention is put into effectwill be understood to a large extent from the aforegoing.

Thus, the material 10, in the form of a molten or plasticized plasticsmass, is fed from the extruder or extruders to the moulding line, viathe circular and separate channels 18, 20 formed in the extrusion nozzle8. At the aforesaid first location, the material is supplied via thefirst channel, wherewith the amount of material supplied, the pressureunder which said pressure medium is supplied through the passages 32,and the vacuum pressure exerting, through the fine passages 34, asuction force on the supplied material, are adapted so that themoulding-jaw recesses corresponding to said intermediate walls 63 aretotally or partially filled at least to the bottom surfaces of saidrecesses. The material layer applied from the first channel 18 ispreferably thinly applied, so as to ensure that the bottoms ofrespective recesses are filled satisfactorily. With certainconfigurations of recesses 62 and intermediate walls 63, air may beenclosed by the material 10 in the bottoms of the recesses of themould-jaws, thereby making it practically impossible to fill therecesses. In this case air-evacuating passages 34 are activated. Inother configurations, such as those for example where the intermediatewalls extend helically around the conduit, the possibiiity of evacuatingsaid air is incorporated in said configuration, thereby greatlyreducing, or eliminating, the need to generate a suction force throughpassages 34. It will be evident herefrom that variances are available inwhich there is utilized solely pressure created through passages 32, ora suction force created through passages 34 or a combination thereof.Additional material 10 is supplied at said second location, via thesecond channel 20. This further material is supplied in such quantitiesthat the grooves in the mould-jaws are totally filled and so that aconduit wall of desired thickness is formed between the intermediatewall 63, this conduit wall constituting the inner surface of the conduitand the bottom surface of the recesses 62. The newly applied material ispressed against the previously applied material and the mould-jaws bypressure medium supplied to the cavity 30 under a given overpressurethrough the nozzles 39. Preferably no vacuum pressures prevail in thepassages 35 during this course of events.With regard to the pressureprevailing in the cavities 29 and 30, the pressure in cavity 30 isnormally greater than the pressure in cavity 29.

For the purpose of smoothing the inner wall 43 of the conduit, theconduit is passed over the smoothing mandril 31, the diameter of thecylindrical part 42 of which corresponds substantially to the desiredinner diameter of the conduit. Effective smoothing can be effected byheating the mandril 31 to a temperature which ensures that the materialwill be held molten or sufficiently plastic in regions adjacent themandril.

In the event that excessive material is supplied in the aforesaid twostages, this excess material can be collected at the juncture betweenthe input end 40 of the smoothing mandril 31 and the cylindrical part 42thereof. The material thus collected can serve as a buffer. In order,among other things, to provide a certain degree of flexibility withregard to the amount of material supplied, by increasing or decreasingthe thickness of the conduit wall, the mouldjaw parts in one chain 2 arearranged for movement towards and away from corresponding jaw parts inthe second chain, under the action of a spring force by means of thedevices 6,7.

The conduit is supplied with coolant through the nozzles 46 in thecooling station 38, in which there is preferably maintained a givenoverpressure.The sealing washers 47 contribute towards maintaining thisoverpressure.

In order to ensure reliable functioning, the mould-jaws 3 are cooled,for example, with freon gas. To this end, known means for cooling themould-jaws are located within the region of at least a part of themoulding line and/or within the region where the smoothing mandril 31 islocated and/or the immediately following region where coolant issupplied to the inner surface of the conduit. The mouldjaw parts 3 aresuitable cooled in a manner to ensure that the material forming thepattern on the conduit wall and the external parts thereof arecompletely or partially solidified when the conduit moves along thesmoothing mandril.

As previously mentioned, the mould-jaws must be freed from the mouldedconduit. This is effected in the lifting zone 56,57 in substantially themanner described with reference to FIGS. 6a and 6b, i.e. the jawsections 3', and 3" are displaced by means of the guide means 59, 60,61, as indicated by the arrows 58 in FIG. 6a, or in some other suitablemanner. Thus, in displacing the jaw sections, the recesses in respectivesections 3', 3"are moved substantially radially in relation to the partof the conduit engaging said respective jaw section, i.e. one quarter ofthe periphery and herewith substantially in the direction in which theintermediate walls extend in the thickness direction of the conduit., Inthis respect, very little clearance, or substantially no clearance isrequired to prevent the configuration of intermediate walls from lockingto respective jaw sections 3', 3" at the location where the conduit partengages said respective jaw section. When only two mould-jaw parts,mould-jaw halves are used, an extremely pronounced clearance or reliefis required, in order to enable the mould-jaws to be drawn free.

Conduits can be manufactured in accordance with the invention at a highproduction rate, in which the conduit can be cooled more rapidly than athick-wall conduit of homogenous wall thickness.

The invention also offers a new possibility to achieve certainproperties at certain parts of the conduit by means of choosingdifferent materials for different parts. Thus, according to theivnention, conduits or pipes for heat exchangers are designed andproduced, such conduit having an outer part, comprising the flanges forstrengthening purposes, made of a highly modified plastics materialproviding strength and good thermal conductivity. The conduit also has apreferably thin, inner part made of substantially unmodified plasticsmaterial providing bood resistance to e.g. chemically aggressivesubstances. The highly modified material comprises fairly large amountsof e.g. mineral, such as clay, particles or metal, such as iron oraluminium, particles.

By means of different materials for different parts of the conduit ofcourse a lot of new products for various applications may e designed andproduced.

Of course single-wall conduits of known design may be produced accordingto the invention using one material or combinations of more than onematerial. Examples of such conduits are single-wall conduits having onlycircumferential ridges spaced along the conduit or having one or morehelically running ridges, all ridges being equally turned.

The invention has been described in the aforegoing with reference tocertain specific proposals and embodiments. It will be understood,however, that further variants and embodiments are conceivable withoutdeparting from the scope of the invention.

For example, the recesses 62 and intermediate walls 63 may haveconfigurations different to those illustrated and described; for exampleeach recess, as seen in FIGS. 7, 9 and 11, may have a cross-sectionalshape corresponding to an equilateral triangle.

It will also be understood that the apparatus and method according tothe invention may employ more than two mould-jaw chains, while thenumber of mould-jaw sections may be greater than four, even though twochains and four jaw sections are to be preferred for machine-technicalreasons.

Furthermore, the geometry of the conduit with regard to the height ofthe intermediate wall 63, i.e. the depth of the recesses 62, in relationto the total wall thicknesss of the conduit and the residual materialthickness between the bottom surfaces of the recesses and the innersurface of the conduit can vary within wide limits in dependence uponthe desired mechanical strength. The extension of the recess across thecylindrical surface of the conduit and the extension in relation to thetotal wall thickness may also be varied within wide limits.

The flexibility of the conduit can be influenced by the configuration ofthe intermediate walls. When the intermediate walls have the form oflongitudinally extending continuous walls, the conduit is resistant tobending.

It should be mentioned that the method and apparatus according to theinvention can also be used for manufacturing conduits not provided,inter alia, with pronounced recesses but with which, when employingconventional manufacturing techniques, difficulties are experienced, ormay be expected to occur when, inter alia, separating the mould-jawparts from the conduit.

In accordance with one preferred embodiment of the invention, the flangering 21 and end piece 22 are joined with the outer mandril 17 by meansof substantially radially extending, spoke-like connecting and spacingelements, which are not included in the sectional view of FIG. 2, theseconnecting and spacing elements ensuring that the outer mandril 17 isfirmly centered in relation to the flange ring 21 and the end piece 22.In accordance with a further preferred embodiment, the spacing elements33, 33' are designed to offer a low resistance to the material 10passing said elements 33, 33'. In this respect, the spacing elements arestreamlined in the flow direction of the material 10, and exhibit sharpend portions, so that the material can be divided at and sealed-offdownstream of the elements 33, while being subjected to but lowresistance. similar spacing elements 33' are also provided in thevicinity of the forward ends of the mandril 17 and 19, as seen in theflow direction of the material, even where lead-throughs for pressuremedium are not required. In this case, spacing elements 33' are suitablyarranged in both the passage 18 and the passage 20, both between theouter jacket 14 and the outer mandril 17, and between the outer mandriland the inner mandril 19, as illustrated in FIG. 3. This enables theouter mandril 17 to be well centered in relation to the outer jacket 14and the two mandrils 17, 19 to be well centered in relation to eachother.

In order to produce a conduit of uniform wall thickness, the speed atwhich the mould-jaw chains are driven is suitable adapted to thequantity of material fed from the extruder per unit of time. When toomuch material is fed from the extruder, the mould-jaw chains are movedoutwardly, provided that said chains are displaceably arranged insubstantially the radial direction of the conduit under manufacture, asillustrated in FIG. 1 by means of the spring-loaded guide rail 5. Thisprovides an indication that excessive quantities of material is beingsupplied, this excess material being compensated for by increasing thespeed of the mould-jaw chains.

Naturally, the amount of material discharged from the extruder can becontrolled in a corresponding manner.

Above it has been stated that more than one extruder may be used. In theexamples given above two stages have been used for producing conduits.However, conduits may be formed in more than two stages by means of amethod according to the invention. This may be achieved by providing athird location at which material is supplied e.g. via a third channelwhich may be obtained by means of a further mandril. In this wayconduits may be produced which comprise more than two differentconcentric parts. The composition of the conduit may be such that onepart is made of a certain material and all the other parts are made ofanother material. Several combinations are possible, e.g. if the twolayers are not weldable or adhered to each other this can be hleped byhaving a third nozzle beteween the first and the second from which aglue-type of plastics is extruded between the layers. Conduits may ofcourse also be produced which are made of only one material but whichare produced in more than two stages.

The invention is not restricted to the aforedescribed variants andembodiments, and modifications can be made within the scope of theaccompanying claims.

I claim:
 1. In a method of manufacturing a single-wall ribbed conduithaving a smooth inner surface from a thermoplastic extrudable materialwherein said material is fed from at least one extruder through anextruder nozzle and into a molding apparatus comprising mould-jaw chainswhich move adjacent one another along a molding line and includemould-jaw parts whose mutually opposing surfaces in the molding linehave a form corresponding to a desired form of an outer surface of theconduit under manufacture, and wherein said material is pressed againsthe surfaces of the mould-jaw along at least a part of the molding lineand wherein the mould-jaw chain are caused to move away from one anothersubsequent to passing the moulding line, the mould-jaw parts therewithreleasing their engagement with the outer surface of the conduit undermanufacture, the improvement comprising:producing the conduit undermanufacture in at least two stages by supplying said extrudable materialto said moulding line to at least two separate locations, a firstlocation and a second location positioned after the first location inthe direction of conduit movement; supplying said material to said firstlocation from a first channel of circular cross-section and forcing suchmaterial against the mould-jaw part by a gaseous medium under pressureand/or suction-drawing such material against said mould-jaw part by avacuum exerted through passages in said mould-jaw parts, while adjustingthe quantity of material supplied and the pressure and/or vacuum at saidfirst location so that grooves in the mould-jaw parts, corresponding tooutwardly protruding portions of the conduit under manufacture, arefilled with said material at least to the bottom thereof; and supplyingadditional extrudable material to said second location from a secondchannel of circular cross-section formed between an external firstmandril and an internal second mandril and discharging such additionalmaterial into a cavity located between an input end of a smoothingmandril and ends of said first and second mandrils facing said smoothingmandril, and forcing such additional material against the materialdelivered to said first location by a pressurized gaseous medium fed tosaid cavity.
 2. In a method as defined in claim 1 wherein said supplyingof extrudable material to said first and second channel involves feedingdifferent extrudable material from at least two extruders to eachrespective channel whereby the conduit under manufacture is providedwith a composite structure and include two different concentricportions, each composed of a different material.
 3. In a method asdefined in claim 1 including guiding said internal second mandril inrelation to said external first mandril at least in the region where theends of said mandrils face said smoothing mandril with the aid ofspacing elements arranged in said second chanel.
 4. In a method asdefined in claim 3 including providing lead-throughs through saidspacing elements for passages which extend transversely through saidsecond channel and through which a pressurized medium and/or vacuum issupplied to said first location, said passage extending from a supply ofpressurized medium and/or a vacuum to a supply channel extending axallywithin said inner mandril.
 5. In a method as defined in claim 3 whereinsaid guiding of said external first mandril in relation to an outerjacket with which said first mandril forms said first channel, occurswith the aid of spacing elements arranged in said first channel.
 6. In amethod as defined in claim 2 including providing said smoothing mandrilwith a softly rounded convex shape at an input end thereof, andmaintaining said smoothing mandril at an adequately high temperature sothat an inner surface of the conduit under manufacture is smoothed bypressure of said smoothing mandril against said inner surface, andcooling the mould-jaws so that the material from which the pattern andsurface parts of the conduit wall are formed, is at least partiallysolidified when the conduit under manufacture moves along said smoothingmandril.
 7. In a method as defined in claim 2 wherein at least twomould-jaw chains are moved parallel with one another along the mouldingline during the formation of the conduit under manufacture, saidmould-jaw chains including mould-jaw parts symetrically divided in thelongitudinal direction of the moulding line such that each of saidmould-jaw parts includes two mould-jaw sections arranged for movementtoward and away form one another and arranged to move adjacent oneanother along the moulding line to form said mould-jaw parts; andsubstantially at the same time as the mould-jaw parts are moved apartafter passing the moulding line, each of said mould-jaw sections of arespective mould-jaw part is moved in a direction away from anothermould-jaw section so that each mould-jaw section is displacedsubstantially radially outwardly in relation to a conduit portion formedin the moulding line by said mould-jaw sections.
 8. In a method asdefined in claim 7 wherein there is provided two mould-jaw chains and inwhich each mould-jaw part includes two mould-jaw section.
 9. In a methodas defined in claim 7 wherein each longitudinal row of mould-jawsections of the mould-jaw chains is driven by a drive chain arrangedadjacent said mould-jaw sections.
 10. In a method as defined in claim 9wherein said drive chains for each mould-jaw section of the mould-jawparts extend around a common rotational axis at least in the vicinity ofthe zone where the mould-jaw sections move away from one another.
 11. Ina method as defined in claim 1 wherein the inner surface of themould-jaw parts are designed to impart to the outer surface of theconduit under manfuacture a pattern comprising rib-like structures whichextend outwardly from the outer cylindrical boundry of said conduit andwhich form a network of relatively thin intermediate walls, the heightof said rib-like structures constituting a substantial part of the totalwall thickness of said condut,
 12. In a method as defined in claim 11wherein said pattern defines a diamond-shaped network, said networkbeing formed in the moulding line by continuous grooves fromed in themould-jaw parts and extending in left-hand and right-hand helices. 13.In a method as defined in claim 11 wherein said pattern defines adiamond-shaped network, said network being formed in the moulding lineby continuous grooves formed in the mould-jaw parts and extending in thelongitudinal direction and the periferal direction of the moulding line.14. In a method as defined in claim 11 wherein said pattern definescircular cross-sectional structures, said circular structures beingformed by substantially radially extending pegs of correspondingcross-sections arranged on the mould jaw parts.
 15. In a method asdefined in claim 11 wherein said pattern defines a regular hexagoncross-sectional structures said hexagon structures being formed bysubstantially radial extending pegs of corresponding cross-sectionsarranged on the mould-jaw parts.
 16. In an apparatus for producing asingle-wall ribbed conduit having a smooth inner surface from anextrudable plastics material, said apparatus comprising at least oneextruder; a plurality of complementary mould-jaws having mutually facingsurfaces with recesses therein corresponding to a desired form of outersurface of the conduit being produced, said mould-jaws being arrangedfor transportation in an endless chain along a select transport distanceof said chain to define a moulding line; an extrusion nozzle coupledwith said extruder for supplying moulten plastics material to saidmoulding line; pressure means for urging the moulten plastics materialagainst said mutually facing surfaces of the mould-jaws along at least apart of the moulding line; cooling means for cooling the conduit beingproduced; separating means for moving the mould-jaws apart subsequent topassage past said moulding line, said mould-jaws being arranged torelease their engagement with the outer surface of the conduit beingproduced; the improvement comprising:at least two stages for supplyingmaterial to the moulding line at least at two mutually separatedlocations, a first location and a second location positioned after thefirst location in the direction of material movement along said mouldingline; adjustment means for regulating the amount of material suppliedand the pressure and/or vacuum applied at said first location in amanner such that the recesses in the mould-jaws are filled with saidmaterial at least to their respective bottom portions; supply means forproviding a pressurized gaseous medium to said first and second locationto press supplied material against the mould-jaws and for providing avacuum to at least said first location to draw supplied material againstthe mould-jaws; passages in said mould-jaws at the bottom of the groovesof each respective recesses connected to said vacuum supply means atleast in the vicinity of said first location; a first channel having acircular cross-section positioned to supply material to said firstlocation, said first channel opening into an annular first space formedbetween an outer first mandril and the mould-jaws and coupled with saidsupply means for pressurized gaseous medium and vacuum; and a secondchannel having a circular cross-section positioned to supply material tosaid second location, said second channel being formed between saidouter said first mandril and an inner second mandril, said secondchannel opening into an annular second space defined, when seen in thedirection of material movement, by respective end of said first andsecond mandrils facing a smoothing mandril located past said secondchannel and an input end of said smoothing mandril; and means forinterconnecting said supply means for pressurized gaseous medium to saidsecond space to provide said pressurized gaseous medium to such secondspace to press the material supplied at the second location against thematerial supplied at the first location.
 17. In an apparatus as definedin claim 16 wherein said smoothing mandril is provided with a softlyrounded convex shape at an input end thereof.
 18. In an apparatus asdefined in 16 wherein spacing elements are arranged in said secondchannel in the region of the ends of said first and second mandrilsfacing the smoothing mandril for guiding said second inner mandril inrelation to said first outer mandril.
 19. In an apparatus as defined inclaim 18 wherein said spacing elements form lead-through extendingthrough walls of said second channel for accomodating passages supplyingpressurized gaseous medium to said first location, said passagesextending transversely from a pressurized gaseous medium supply channelextending axally within the first mandril.
 20. In an apparatus asdefined in claim 16 wherein spacing elements are also provided in saidfirst channel for guiding said first mandril in relation to an outersleeve, said outer sleeve forming, together with said first mandril,said first channel.
 21. In an apparatus as defined in claim 16 whereinthe smoothing mandril includes means for heating said mandril to atemperature such that the material on the inner surface of the conduitformed in said moulding line is held in a plasticized state to an extentsuch that the inner surface thereof can be substantially smoothened bysliding action with the smoothing mandril.
 22. In an apparatus asdefined in claim 18 wherein the spacing elements are arranged to offerrelatively low resistance to the material passing said spacing elements,said spacing elements being streamlined in the flow direction ofmaterial and having relatively sharp end portions such that the materialis divided and sealed off subsequent to passing said elements under lowresistance.
 23. In an apparatus as defined in claim 16 wherein saidmoulding line includes by at least two mutually parallel mould-jawchains, said mould-jaw chains including mould-jaw parts symetricallydivided in the longitudinal direction of the moulding line such thateach mould-jaw part includes at least two mould-jaw sections which arearranged for movement toward and away from one another and to moveadjacent one another to form said mould-jaw parts in said moulding line.24. In an apparatus as defined in claim 23 wherein means co-acting withthe mould-jaw chains are provided whereby each mould-jaw section of themould-jaw parts is arranged to move in a direction away from anothermould-jaw section subsequent to passing said moulding line substantiallyat the same time as the mould-jaw parts are moved apart, so that eachmould-jaw section is moved substantially radially outwards in relationto the conduit portion formed in the moulding line by said mould-jawsections.
 25. In an apparatus as defined in claim 23 wherein twomould-jaw chains are provided and each mould-jaw part includes twomould-jaw sections.
 26. In an apparatus as defined in claim 23 whereineach longitudinal row of mould-jaw sections of said mould-jaw chains isprovided with a drive chain for driving said chain of mould-jaws.
 27. Inan apparatus as defined in claim 26 wherein the drive chains for themould-jaw sections of each mould-jaw part extend around a commonrotational axis at least in the vicinity of the zone where saidmould-jaw section are arranged to move away from one another and awedge-shaped means is positioned at said zone for moving said sectionsaway from one another by acting against the drive chains.
 28. In anapparatus as defined in claim 27 wherein said wedge-shaped meanscomprises guide rails co-acting with said mould-jaw sections.
 29. In anapparatus as defined in claim 23 wherein said mould-jaw sections of eachmould-jaw part are guided relative to one another when moved towards oraway from each other by at least one guide pin arranged adjacent the onemould-jaw section, said pin having a narrowing and generally rectangularcross-section, with a corresponding recess in an adjacent mould-jawsection.
 30. In an apparatus as defined in claim 16 wherein the innersurface of the mould-jaw parts are designed such as to impart to theouter surface of the conduit under manufacture, a pattern comprisingrib-like structures which extend substantially radially outwards fromthe outer cylindrical boundry of such conduit and which formtherebetween a system of relatively thin intermediate walls, where theheight of said rib-like structures corresponds to a substantial part ofthe wall thickness of said conduit.
 31. In an apparatus as defined inclaim 30 wherein said mould-jaw parts include continuous grooves forforming said intermediate walls, said grooves being arranged to form aleft-hand and right-hand helices in the direction of the moulding lineand thereby define a diamond-shaped network of intermediate walls onsaid conduit.
 32. In an apparatus as defined in claim 30 wherein saidmould-jaw parts include continuous grooves for forming said intermediatewalls, said grooves being arranged to extend in the longitudinaldirection and the peripheral direction of the moulding line and therebydefine a diamond-shaped network of intermediate outer walls in saidconduit.
 33. In an apparatus as defined in claim 30 wherein saidmould-jaw parts include outwardly projecting pegs for forming saidrib-like structures, said pegs having a cross-sectional shape ofgenerally circular configuration.
 34. In an apparatus as defined inclaim 30 wherein said mould-jaw parts include outwardly projecting pegsfor forming said rib-like structures, said pegs having a cross-sectionalshape corresponding substantially to a regular hexagon configuration.35. Apparatus for producing a single-wall ribbed conduit having a smoothinner surface from an extrudable plastics material, comprising:A. a pairof endless chains of complementary mouldjaws formed of a plurality ofmould-jaw parts having interior moulding surfaces, said chains beingarranged to move through a given distance in close proximity to oneanother, said distance defining a moulding line; B. at least oneextrusion nozzle positioned at an input end of said moulding line, sidextrusion nozzle being operationally coupled to at least one extruderproviding a stream of extrudable material to said nozzle; C. a first,outer mandril positioned within said nozzle so as to define a firstchannel of circular cross-section for guiding extrudable material to afirst location in said moulding line, and a second, inner mandrilpositioned inwardly of said first mandril so as to define a secondchannel of circular cross-section for guiding extrudable material to asecond location in said moulding line, said second location being spacedfrom sid first location and positioned after the first location in thedirection of material movement;said interior moulding surfaces of eachmould-jaw part being provided with a plurality of recesses defining aribbed outer surface of the conduit being produced, each of saidrecesses having a relatively fine passage at a bottom thereof forpassage of a gaseous medium therethrough; D. vacuum means operationallycoupled at said first location with the interior moulding surfaces ofsaid mouldjaw parts to extract gaseous medium from at least the bottomof said recesses in said mould-jaw parts and draw extrudable material toat least the bottom of each recess; E. pressure means operationallycoupled at said first location with the interior moulding surface ofsaid mouldjaw parts to provide a pressurized fluid media to a spacebetween said moulding surfaces to force material at said first locationat least partially into the recesses in the mould-jaw parts; F. asmooothing mandril having a rounded convex surface facing said first andsecond mandrils and a cylindrical portion joined with said roundedsurface, said cylindrical portion being located in close proximity tothe moulding surfaces of opposing mould-jaw parts, said smoothingmandril being positioned within said said second location and spacedfrom said second channel; and G. fluid-coupling means interconnectingsaid pressure means with said second locatin at least at the spacebetween said second channel and said smoothing mandril to provide apressurized fluid medium to said space and force material at said secondlocation against the material supplied at the first location.